The present invention relates to a freezing device constituted of a single refrigerant circuit in which a refrigerant discharged from a compressor is condensed and then evaporated to exert a cooling function.
Heretofore, a so-called Freon has broadly been used as a refrigerant for use in a refrigerator or a freezer. However, a specific Freon containing chlorine destroys ozone in an upper region of the atmosphere, many Freons influence global warming as a greenhouse-effect gas, and the Freon is an object whose use is restricted. Therefore, there is demanded the development of a replaceable refrigerant composition capable of maintaining the performance of the refrigerant without any danger that the ozone layer is destroyed and without changing a conventional freezing circuit.
The refrigerant which can be used is required to have characteristics that physical properties such as its composition and boiling point do not change during vaporization or condensation of the gas and that compatibility with an oil (an alkyl benzene) for use as a lubricant is high. In addition, it is required that the boiling point of the refrigerant be sufficiently low in consideration of a relation between the boiling point and a targeted temperature in chamber and that a critical temperature be high for a smooth operation under a room-temperature environment. Therefore, it is very difficult that such requirements are satisfied by the refrigerant gas of a single component.
To solve the problem, a mixed refrigerant constituted of two or more of components has heretofore been used, and properties such as the boiling point of the mixed refrigerant are adjusted by selecting the components to be mixed. Especially, to make it possible to realize an extremely low temperature below −80° C. in chamber, a non-azeotropic mixed refrigerant constituted of two or more of components is used. It is difficult to liquefy components each having a low boiling point and a low critical temperature by a capability of a condenser which operates under the room-temperature environment, and therefore, a multistage system or the like is employed in which the refrigerant components are condensed in multiple stages.
However, in the multistage freezing system, a structure is complicated and enlarged, and maintenance becomes difficult, which leads to a problem that a running cost remarkably increases.
To solve the problem, heretofore, there have been developed a non-azeotropic mixed refrigerant containing butane, ethylene and R14 (carbon tetrafluoride: CF4) (see Japanese Patent Application Laid-Open No. 2003-13049), and a non-azeotropic mixed refrigerant containing butane, ethane and R14 (see Japanese Patent Application Laid-Open No. 2003-13050). According to these non-azeotropic mixed refrigerant gases, a treating property of the refrigerant in the freezer is secured by an operability of butane having a high boiling point at normal temperature, and the extremely low temperature is realized by ethane or ethylene having a remarkably low boiling point. In consequence, the temperature in chamber can be set to −60° C. or less without using any complicated multistage freezing system.
However, in the above-described conventional non-azeotropic mixed refrigerant, a combustible gas such as ethylene or ethane is used. Especially, to realize an extremely low temperature of −60° C. or less in chamber, at least a mixture ratio of ethane or ethylene in a mixture of butane and ethane or ethylene has to be 10% or more, and a state of the non-azeotropic mixed refrigerant remains in a combustible region. Therefore, there has been a problem that the combustible mixed refrigerant has an unsatisfactory reliability in respect of safety, and has a very unsatisfactory treating property.